The present disclosure relates generally to semiconductor manufacturing. Specifically, the present disclosure relates to devices with moving members and methods of making the same.
Micro-electro-mechanical systems (MEMS) devices include very small electro-mechanical systems incorporated into semiconductor IC circuits. Some MEMS devices have moving members separated from the substrate by a cavity and one or more bumps. The bumps may be on top of the substrate or on the moving members. The purpose of the bumps is to constrain the range of motion of the moving members, preventing the moving members from touching the substrate or layers directly on top of the substrate. When some materials, such as oxides, are used for bumps, the oxide bump can cause dielectric charging issue. Additionally, the resulting device with oxide bump may experience parasitic capacitance and stiction. Parasitic capacitance and stiction may make the resultant device ineffective in sensing some movement.
Some conventional techniques for forming the cavity include depositing a sacrificial oxide layer over the substrate. The sacrificial oxide layer is then patterned and etched away using vapor Hydrogen Fluoride (HF) as an etchant to create a cavity. Other conventional techniques may use plasma dry etching or chemical wet etching. A silicon wafer is then fusion bonded onto the etched sacrificial oxide layer. Moving members are created using the fusion bonded silicon wafer, and the cavity provides space for motion of the moving member. However, this conventional technique can be less than optimal in some circumstances. For instance, the fusion bond between the silicon wafer and the oxide layer may be undesirably weak if the silicon wafer is deposited after the oxide layer is etched.
As another example, one conventional process forms the sacrificial layer without planarization. Unevenness in the sacrificial oxide layer then creates step shapes in the cross section of the silicon layer that is formed on top of the sacrificial layer. These step shapes generally provide a poor mechanical boundary and can result in low yield.
In another example, the sacrificial oxide layer is formed and planarized. However, it can be difficult to control the dimension of the cavity as it is formed during HF etching. Therefore, what is needed is an improved MEMS device and method for making the same.